I rode down the CA-281 on the way to Potes from the Santander ferry. The climb up to the reservoir is spectacular
I arrive in Santander at 2.15pm and will probably head to the Parador at Cervera de Pisuerga for the first night. That looks like a rather splendid road to head down to get there. Starts nice and wide at the northern end then gets tight and narrow through that gorge.